IMS – IP Multimedia Subsystem

Architecture, nodes, flows and use cases - why IMS is better?

Sisukord

1. Kommunikatsioonimeedia ja Lisaväärtusteenused, SDP-d.

2. IMS. Arhitektuur ja võrgusõlmed

3. Sessiooni haldus (Internet)

4. Sessiooni haldus (IMS)

1. Kommunikatsioonimeedia

• Internet• (Mobiil, fix) sidevõrgud

Sidevõrgu komponendid:• Juurdepääsuvõrk (raadio, fikseeritud liin jne)• Tuumvõrk (ahelkommuteeritud ja pakettside)• Teenusvõrk

• Standard- ja lisaväärtusteenused (VAS) (mobiil-) sidevõrgus.

1. Value-Added-Services 1• Taditional IN (Intelligent Network)

• The Intelligent Network (IN), is a network architecture intended both for fixed as well as mobile telecom networks. It allows operators to differentiate themselves by providing value-added services in addition to the standard telecom services such as PSTN, ISDN and GSM services on mobile phones.

• In IN, the intelligence is provided by network nodes on the service layer, distinct from the switching layer of the core network, as opposed to solutions based on intelligence in the core switches or telephone equipments.

• IN is based on the Signaling System #7 (SS7) protocol between telephone network switching centers and other network nodes owned by network operators.

CS: ISUP + IN protocols (CAMEL; INAP CS1(+)).

1. Value-Added-Services 2• Parlay/OSA is an open API for the telephone network. • developed by The Parlay Group, which works closely with ETSI and 3GPP,

which all co-publish it. Within 3GPP• Parlay is part of Open Services Architecture. • The Parlay Group [1] is a technical industry consortium (founded 1998) that

specifies APIs for the telephone network. These APIs enable the creation of services by organizations both inside and outside of the traditional carrier environment. In fact, it is hoped that services can be created by IT developers, rather than telephony experts.

• Important Parlay APIs include: call control, conferencing, user interaction (audio and text messaging, SMS/MMS), and billing.

• The APIs are specified in the CORBA Interface definition language and WSDL. The use of CORBA enables remote access between the Parlay gateway and the application code.

• A set of Java mappings allow the APIs to be invoked locally as well. A major goal of the APIs is to be independent of the underlying telephony network technology (e.g. CDMA, GSM, landline SS7).

1. Value-Added-Services 3

IN teenused: - Lühinumbri ja privaatnumbriplaani teenused (VPN).

Huntgroup.- Maksustamislahenduse (Charging System)

IN + Parlay teenused:- Personaalne suunamine,blacklist jne.- Kompanii sekretäri / põhinumbri lahendus- Huntgroup- Click2Dial, GroupCall- UserStatus

1. Need for IMSIMS –> Internet services everywhere and at any time using cellular

technologies.

1. Merging internet and cellular world.• 3G: Circuit (2G) and Packet-Switched.• Fix: ISDN, DSL

2. QoS – charging, interrogation of different services. PS – best-effort QoS (VoIP).3. Change Multimedia Sessions (session control etc)

General:- IMS does not mandate any particular Business Model- Defines standard interfaces for service developers- Creates Service environment – service can access any aspect of the

session (instead of completely independent services). SDE.- IMS does not depend on the CS domain

2. IMS

• The IP Multimedia Subsystem (IMS) is an architectural framework for delivering Internet Protocol (IP) multimedia services.

• To ease the integration with the Internet, IMS uses IETF protocols wherever possible, e.g. Session Initiation Protocol (SIP).

• IMS is not intended to standardize applications but rather to aid the access of multimedia and voice applications from wireless and wireline terminals

• Horizontal control layer that isolates the access network from the service layer. From a logical architecture perspective, services need not have their own control functions, as the control layer is a common horizontal layer. However in implementation this does not necessarily map into greater reduced cost and complexity.

2. History

• IMS was originally defined by an industry forum called 3G.IP, formed in 1999. • 3G.IP developed the initial IMS architecture, which was brought to the 3rd Generation

Partnership Project (3GPP), as part of their standardization work for 3G mobile phone systems in UMTS networks.

• It first appeared in Release 5 (evolution from 2G to 3G networks), when SIP-based multimedia was added. Support for the older GSM and GPRS networks was also provided.

• 3GPP2 (a different organization than 3GPP) based their CDMA2000 Multimedia Domain (MMD) on 3GPP IMS, adding support for CDMA2000.

• 3GPP release 6 added interworking with WLAN.

• 3GPP release 7 added support for fixed networks, by working together with TISPAN release R1.1, the function of AGCF(Access Gateway control function) and PES (PSTN Emulation Service) are introduced to the wire-line network for the sake of inheritance of services which can be provided in PSTN network.

2. IMS requirements

(3GPP IMS R5) Support for: • Establishing IP Multimedia Sessions.• Mechanism to negotiate QoS.• Interworking with internet and CS networks.• Roaming.• Strong Service Control Imposed by operator with respect

to the services delivered to the end user.• Rapid service creation without requiring standardization.

• R6 –> Multiple access

2. IMS Protocols

Signalling Plane - Session Control Protocol - SIP (Session Initiation Protocol, RFC 3261 by IETF):

• Protocol to establish and manage Multimedia sessions over IP networks.

• Follows client-server model• Design borrowed from SMTP and HTTP• Text-based(CS analogy – ISUP + IN protocols)

Authentication, Authorization, Accounting – Diameter (Radius based)

Media Plane: • H.248 (signaling nodes to control nodes)• RTP (RealTime Transport Protocol) – RT Media transport – A&V

2. IMS Architecture

IP Multimedia Core Network Subsystem:• 1…n User Databases HSS (Home Subscriber Servers)

ja SLF (Subscriber Location Function)• 1…n SIP servers – CSCF (Call Session Control

Functions)• 1…n AS servers – Application Servers• 1...n MRF (Media Resource Function), each one divided

into MRFC (… Controllers)• 1…n BGCF (Breakout Gateway Control Function)• 1…n PSTN Gateways decomposed to SGW, MGCF and

MGW

2. IMS FunctionsIP Multimedia Core Network Subsystem • Collection of different functions, linked by standardized interfaces, which grouped form one IMS

administrative network. • A function is not a node (hardware box): an implementer is free to combine 2 functions in 1 node,

or to split a single function into 2 or more nodes. • Each node can also be present multiple times in a single network, for dimensioning, load

balancing or organizational issues.

Access network (Juurdepääsuvõrk)

• User can connect to an IMS network in various ways, most of which use the standard Internet Protocol (IP).

• IMS terminals (such as mobile phones, (PDAs) and computers) can register directly on an IMS network, even when they are roaming in another network or country (the visited network).

• The only requirement is that they can use IP and run Session Initiation Protocol (SIP) user agents.

• Fixed access (DSL, cable modems, Ethernet), mobile access (e.g. W-CDMA, CDMA2000, GSM, GPRS) and wireless access (e.g. WLAN, WiMAX) are all supported.

• Other phone systems like PSTN and non IMS-compatible VoIP systems, are supported through gateways.

• From Release 8 onwards, the 2G or 3G circuit-switched network can also be used as an access network to the IMS, if ICS (IMS Centralized Services) is supported

2. IMS Architecture 2

2. IMS Subscriber Management• The Home Subscriber Server (HSS), or User Profile Server Function (UPSF), is a master user

database that supports the IMS network entities that actually handle calls. It contains the subscription-related information (subscriber profiles), performs authentication and authorization of the user, and can provide information about the subscriber's location and IP information. It is similar to the GSM Home Location Register (HLR)

• A Subscriber Location Function (SLF) is needed to map user addresses when multiple HSSs are used. Both the HSS and the SLF communicate through the Diameter protocol (AAA operations).

User identities. 3GPP networks use the following identities:• International Mobile Subscriber Identity (IMSI) • Temporary Mobile Subscriber Identity (TMSI) • International Mobile Equipment Identity (IMEI) • Mobile Subscriber ISDN Number (MSISDN)

• IMS also requires IP Multimedia Private Identity (IMPI) and IP Multimedia Public Identity (IMPU). Are Uniform Resource Identifier (URIs), that can be digits (a Tel URI, like tel:+1-555-123-4567) or alphanumeric identifiers (a SIP URI, like sip:john.doe@example.com).

• The HSS subscriber database contains, the IMPU, IMPI, IMSI, and MSISDN, subscriber service profiles, service triggers and other information.

2. Call/session control 13 types of Call Session Control Function (CSCF) are used to process SIP signalling packets in the

IMS.Proxy-CSCF (P-CSCF)

• SIP proxy that is the first point of contact for the IMS terminal• It can be in the visited network (in full IMS networks) or in the home network (when the visited

network isn't IMS compliant yet). • Some networks may use a Session Border Controller for this function. The terminal discovers its

P-CSCF with either DHCP, or it is assigned in the PDP Context (in General Packet Radio Service (GPRS)).

– it is assigned to an IMS terminal during registration, and does not change for the duration of the registration

– it sits on the path of all signalling messages, and can inspect every message – it authenticates the user and establishes an IPsec security association with the IMS

terminal. This prevents spoofing attacks and replay attacks and protects the privacy of the user. Other nodes trust the P-CSCF, and do not have to authenticate the user again.

– it can also compress and decompress SIP messages using SigComp, which reduces the round-trip over slow radio links

– it may include a Policy Decision Function (PDF), which authorizes media plane resources e.g. quality of service (QoS) over the media plane. It's used for policy control, bandwidth management, etc. The PDF can also be a separate function.

– it also generates charging records

2. Call/session control 2• Serving-CSCF (S-CSCF) is the central node of the signalling plane. • It is a SIP server, performs session control. • It is always located in the home network. • It uses Diameter Cx and Dx interfaces to the HSS to download and upload

user profiles — it has no local storage of the user. All necessary information is loaded from the HSS.

– it handles SIP registrations, which allows it to bind the user location (e.g. the IP address of the terminal) and the SIP address

– it sits on the path of all signaling messages, and can inspect every message – it decides to which application server(s) the SIP message will be forwarded, in

order to provide their services – it provides routing services, typically using Electronic Numbering (ENUM)

lookups – it enforces the policy of the network operator – there can be multiple S-CSCFs in the network for load distribution and

high availability reasons. It's the HSS that assigns the S-CSCF to a user, when it's queried by the I-CSCF.

2. Call/session control 3

• An Interrogating-CSCF (I-CSCF) - another SIP function located at the edge of an administrative domain.

• Its IP address is published in the Domain Name System (DNS) of the domain so that remote servers can find it, and use it as a forwarding point (e.g. registering) for SIP packets to this domain.

• The I-CSCF queries the HSS using the Diameter Cx interface to retrieve the user location (Dx interface is used from I-CSCF to SLF to locate the needed HSS only), and then routes the SIP request to its assigned S-CSCF.

• Up to Release 6 it can also be used to hide the internal network from the outside world (encrypting part of the SIP message), in which case it's called a Topology Hiding Inter-network Gateway (THIG).

• From Release 7 onwards this "entry point" function is removed from the I-CSCF and is now part of the Interconnection Border Control Function (IBCF). The IBCF is used as gateway to external networks, and provides NAT and Firewall functions (pinholing).

2. Application Servers• Application servers (AS) host and execute services, and interface with the

S-CSCF using Session Initiation Protocol (SIP). • Depending on the actual service, the AS can operate in SIP proxy mode,

SIP UA (user agent) mode or SIP B2BUA (back-to-back user agent) mode. • An AS can be located in the home network or in an external third-party

network. • If located in the home network, it can query the HSS with the Diameter Sh

interface (for a SIP-AS) or the Mobile Application Part (MAP) interface (for IM-SSF).

• SIP AS: native IMS application server • IP Multimedia Service Switching Function (IM-SSF): an IM-SSF interfaces

with Customized Applications for Mobile networks Enhanced Logic (CAMEL) Application Servers using Camel Application Part (CAP)

• Open Service Access-Service Capability Server (OSA-SCS): Interface to the OSA framework Application Server

2. Application Servers 2

2. Media Servers & Gateways

• The Media Resource Function (MRF) provides media related functions such as media manipulation (e.g. voice stream mixing) and playing of tones and announcements.

• The MRFC is a signalling plane node that acts as a SIP User Agent to the S-CSCF, and which controls the MRFP across an H.248 interface

• PSTN Gateways.

2. NGN & IMS

• Next Generation Networks (NGN)

• IMS as a subsystem of Service Layer of NGN concept

• IMS is a standardised NGN architecture for an Internet media-services capability

3. Internet Session Control

Signalling Plane – Session Control

SIP Functionality:• Session Description & SDP (Session Transcription

Protocol).• Offer/Answer Model.• SIP ja SIP URIs.• UserLocation.

SIP Defines Entities: User Agents, Proxy Servers, Redirect Servers

3. SIP Statuscode ranges

3. SIP Methods

3. SIP Transactions 1

3. SIP Transactions 2

3. SIP Transactions 3 - SigComp

3. SIP NAT Traversal 1

3. SIP NAT Traversal 2

3. SIP NAT Traversal 3

3. SIP NAT Traversal - Skype

4. Registration

4. IMS Session Control - Phonecall

4. AS as SIP UA 1

4. AS as SIP UA 2

4. AS as SIP UA 3

4. AS as SIP proxy 1

4. AS as SIP proxy 2

4. AS as SIP B2B UA 1

4. AS as SIP B2B UA 2

Kasutatud kirjandus

• [1] Camarillo, Garcia-Martin, The 3G IP Multimedia Subsystem

• [2] Wikipedia: “IMS”, “IN”, “Parlay OSA” “NGN”

• [3] http://freshmeat.net/articles/nat-traversal-for-the-sip-protocol